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Iron in the Earth’s core weakens before melting

11 October 2013

The iron in
the Earth’s inner core weakens dramatically before it melts, explaining the
unusual properties that exist in the moon-sized solid centre of our planet that
have, up until now, been difficult to understand.

Scientists
use seismic waves - pulses of energy generated during earthquakes - to measure
what is happening in the Earth’s inner core, which at 6000 km beneath our feet
is completely inaccessible.

Problematically
for researchers, the results of seismic measurements consistently show that
these waves move through the Earth’s solid inner core at much slower speeds
than predicted by experiments and simulations.

Specifically,
a type of seismic wave called a ‘shear wave’ moves particularly slowly through
the Earth’s core relative to the speed expected for the material – mainly iron
– from which the core is made. Shear waves move through the body of the object
in a transverse motion - like waves in a rope, as opposed to waves moving
through a slinky spring.

Now, in a
paper published in Science Express,
scientists from UCL have proposed a possible explanation. They suggest that the
iron in the Earth’s core may weaken dramatically just before melting, becoming
much less stiff. The team used quantum mechanical calculations to evaluate the
wave velocities of solid iron at inner-core pressure up to melting.

The proposed mineral models for the inner core have always shown a faster wave speed than that observed in seismic data. This mismatch has given rise to several complex theories about the state and evolution of the Earth’s core.

Professor Lidunka Vočadlo

They
calculated that at temperatures up to 95% of what is needed to melt iron in the
Earth’s inner core, the speed of the seismic waves moving through the inner
core decreases linearly but, after 95%, it drops dramatically.

At about 99% of the melting temperature of iron, the team’s
calculated velocities agree with seismic data for the Earth’s inner core. Since
independent geophysical results suggest that the inner core is likely to be at
99-100% of its melting temperature, the results presented in this paper give a
compelling explanation as to why the seismic wave velocities are lower than
those predicted previously.

Professor
Lidunka Vočadlo, from the UCL department of Earth Sciences and an author of the
paper said: “The Earth’s deep interior still holds many mysteries that
scientists are trying to unravel.

“The
proposed mineral models for the inner core have always shown a faster wave
speed than that observed in seismic data. This mismatch has given rise to
several complex theories about the state and evolution of the Earth’s core.”

The authors
stress that this is not the end of the story as other factors need to be taken
into account before a definitive core model can be made. As well as iron, the
core contains nickel and light elements, such as silicon and sulphur.

Professor Vočadlo
said: “The strong pre-melting effects in iron shown in our paper are an
exciting new development in understanding the Earth’s inner core. We are
currently working on how this result is affected by the presence of other
elements, and we may soon be in a position to produce a simple model for the
inner core that is consistent with seismic and other geophysical measurements. ”